Virtually all animals can alter their behavior based on past experience. What underlies this ability to acquire, store, and retrieve information is synaptic plasticity, whereby existing connections among neurons are strengthened or weakened and new synapses are formed or existing ones removed. The capacity for synaptic plasticity and, by consequence, for learning and memory is not constant throughout life; it often peaks relatively soon after birth and then typically declines, at variable rates, with increasing age. In many brain systems and animals, there are distinct phases of greatly enhanced plasticity for specific sensory experiences or sensory-motor interactions. Neuronal plasticity is particularly prominent in the developing brain. On the other hand, synaptic plasticity in the adult brain is widespread and is a key feature of many brain regions, like the hippocampus, the striatum, or the cerebellum. Thus, although neuronal plasticity is certainly much more profound in the developing brain than in adulthood, it is not exclusively restricted to that period.
Acoustic experiences change cortical maps in the auditory cortex (ACx), and these changes are required for auditory cognition1-3. During an early critical period (a few neonatal days in rodents), passive exposure to a tone of certain frequencies expands the ACx areas that are tuned to those frequencies4-7. This cortical map plasticity is restricted in adults3,8,9. Restrictive mechanisms that control the duration of the early critical period and impede cortical map plasticity in adults are still being debated2,10-14. 
In rodents, the early critical period for ACx map plasticity and thalamocortical (TC) connectivity is restricted to a few early postnatal (P) days4. Long-term synaptic plasticity, in the form of long-term potentiation (LTP) or long-term depression (LTD), at TC projections has been implicated as a cellular mechanism of cortical map plasticity in the ACx. TC LTP and LTD are also restricted to the early critical period15-17. Like ACx map plasticity8,18,19, LTP and LTD can be unmasked at TC projections in adults if the activation of TC projections is paired with that of cholinergic projections emanating from the nucleus basalis10,15,16.
Adenosine, which is released in an activity-dependent manner20-22 and is a negative regulator of neurotransmitter release at excitatory synapses through activation of A1 adenosine receptor (A1R)23, appears to be an important intermediate of cholinergic modulation of TC synaptic plasticity10.
The adenosine A1 receptor (A1R) is one member of the adenosine receptor group of G protein-coupled receptors with adenosine as endogenous ligand. A1R is widely distributed throughout the central nervous system (CNS), with the highest levels occurring in the cerebral cortex, hippocampus, cerebellum, thalamus, brain stem, and spinal cord of the rat. A1R is coupled to pertussis toxin-sensitive Gi-proteins to inhibit adenylate cyclase. The most prominent effect of A1R on the brain is to depress excitatory transmission. At the presynaptic site, A1R activation inhibits synaptic transmission by the suppression of N-type calcium channels and by a direct downregulation of the release apparatus. At postsynaptic sites, A1Rs are located in the postsynaptic density where they can influence the responsiveness to excitatory stimuli by a simultaneous control of N-type calcium channels and N-methyl-D-aspartate receptors (NMDARs). In addition, A1R in neuronal cells is also located nonsynaptically where activation of A1Rs results in G-protein-dependent activation of inwardly rectifying K+ channels (GIRKs), leading to hyperpolarization of the resting membrane potential. Thus, the A1R can affect neuronal excitability and control of “basal” synaptic transmission (i.e., under conditions where synaptic plasticity is not engaged) by the activation of A1R located presynaptically and postsynaptically as well as nonsynaptically. Reviewed, e.g., in Chen, Int. Rev. Neurobiol., 2014, 119:257-307, Ch. 12.
Ecto-5′-nucleotidase (Nt5e; EC 3.1.3.5) catalyzes the conversion of purine 5′-mononucleotides to nucleosides, the preferred substrate being AMP. The enzyme consists of a dimer of 2 identical 70-kD subunits bound by a glycosyl phosphatidyl inositol linkage to the external face of the plasma membrane.